This invention relates to a brake system for automotive vehicles.
A brake system of a certain kind for automotive vehicles includes a booster unit which is disposed between the brake pedal and the master cylinder to amplify the brake pedal depression force so that sufficient braking force relative to the maximum speed and the weight of the vehicle can always be applied to the brakes and the brake pedal depression force can be reduced to alleviate the fatigue of the vehicle driver. In a booster unit employed hitherto in this field, there is a fixed relation between the brake pedal depression force and the amplified output of the booster unit. In a vehicle, especially, a commercial vehicle such as a truck, however, there is a great difference between the weight of the vehicle in a no-loaded condition and that in a full-loaded condition. It is therefore necessary to change the brake pedal depression force depending on the weight of the vehicle in order that the same deceleration can be attained in both of the no-loaded and full-loaded conditions. If the output of the booster unit were so set that sufficient braking force can be obtained even in a heavy-loaded condition of the vehicle, application of a small depression force to the brake pedal in a light-loaded condition of the vehicle would give rise to locking of the wheels thereby impairing the stability of braking. Further, since the brake pedal depression force and its reaction force are proportional to each other, a large reaction force increasing the fatigue of the driver will appear in a heavy-loaded condition of the vehicle.
Also, in such a brake system it is already known to provide a pressure control valve in the brake circuit connected to the rear brakes in order that the rear wheels may not be locked earlier than the front wheels when braked due to a shift of the load. The function of the pressure control valve is such that, as soon as the rear-wheel braking force exceeds a certain predetermined setting, the rate of increase of the rear-wheel braking force is reduced to lower than that of the front-wheel braking force. In a pressure control valve employed commonly hitherto in this field, the transition point at which the rate of increase of the rear-wheel braking force starts to be slowed down, is generally fixed. However, in order to maintain a high braking efficiency without causing locking of the rear wheels regardless of the weight of the vehicle, it is also necessary to change the distribution of the rear-wheel braking force to the front-wheel braking force depending on the weight of the vehicle.